Ising Supercriticality and Universal Magnetocalorics in Spiral Antiferromagnet Nd${}_3$BWO${}_9$

Abstract

The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of a ferromagnet has long been a cornerstone for understanding universality of phase transitions and critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the spiral Ising compound Nd${}_3$BWO${}_9$ with kagome layers. In its phase diagram, we identify a metamagnetic transition line with a critical endpoint (CEP) located at $\hat{I}{¼}_0{H}_{\mathrm{c}}\simeq 1.04$ T and $T_{\mathrm{c}}\simeq 0.3$ K. Above the CEP, an Ising supercritical regime emerges with crossover lines that follow a universal scaling law, as evidenced by the specific heat, magnetic susceptibility, and magnetocaloric measurements. Remarkably, we observe highly sensitive magnetic cooling near the emergent CEP, characterized by a divergent magnetic Grüneisen ratio $\hat{I}{“}_H\propto 1/t^{{\hat{I}}^2+{\hat{I}}^3-1}$, with ${\hat{I}}^2+{\hat{I}}^3\simeq 1.563$ the sum of critical exponents of the 3D Ising universality class and $t\equiv (T-T_{\mathrm{c}})/T_{\mathrm{c}}$ the reduced temperature. Adiabatic demagnetization from 2 K and 4 T reaches a minimum temperature of 195 mK, via a self-cascading process that combines supercritical and topological cooling. Our findings open a new avenue for studying supercritical phenomena and magnetic refrigeration with the frustrated rare-earth compounds RE${}_3$BWO${}_9$ and, more broadly, in Ising-anisotropic antiferromagnets such as spin ices.

5 pages, 4 figures